Single acting and Double acting Compressors

In single acting compressors, air is drawn and compressed on only one side of the piston.  When the piston moves down, the suction valve opens and air enters the cylinder.  When the piston moves up, at the end of the compression, the discharge valve opens and the compressed gas is released.

The other side of the piston is connected to the crankshaft. 

In a double acting compressor, air is drawn and compressed on both sides of the piston.  The piston is operated by a connecting rod through an airtight seal.  There are compression chambers on both sides of the piston and a set of suction and discharge valves.  .  

Double acting compressors have higher efficiency.  However, they are expensive to manufacture. They are usually water cooled.